Dilution burner

ABSTRACT

A dilution burner comprising at least two separate injectors with different injection cross-sections, for a fuel gas and a comburent gas, opening into the entry of a combustion chamber, means being provided in order that, when the injection flow of gas in the combustion chamber drops below a value preset in terms of the minimum speed of the gases needed at the outlet of the injectors in order to prevent a flash back, the injector with the largest cross-section shall be cut out of service, the supply of the combustion chamber being then solely carried out by the remaining injector or injectors.

United States Patent Baguet Sept. 2, 1975 54 1 DlLUT/ION BURNER FOREIGNPATENTS OR APPLICATIONS [75] Inventor: Paul Baguet, Brussels, Belgium508,850 7/1971 Switzerland [73] Assignee: Compagnie dEtudes et de 1'4655/1969 f f Participations Industrielles, M81445 4/1970 United KingdomGrand Rue Luxembur 1,138,699 1/1969 United Kingdom g 1,810,976 6/1970Germany 22 Filed; Man 6, 97 1,918,896 /1969 Germany [21] Appl' N07338468 Primary Examiner-Carroll B. Dority, Jr. [30] Foreign ApplicationPriority Data Attorney, Agent, or FirmSughrue, Rothwell, Mion,

Zinn & Macpeak Mar. 16, 1972 Luxemburg ..64978 [52] US. Cl. 431/61;431/90; 431/115',

431/158; 431/284 [57] ABSTRACT [51 Int. C1. F23Q 9/08 A dilution burnerp sing at ast two separate in- [58] Field of Search 431/30, 31, 61, 89,90, jectors with different injection cross-sections, for a 431/115, 158fuel gas and a comburent gas, opening into the entry of a combustionchamber, means being provided in [56] References Cited order that, whenthe injection flow of gas in the com- UNITED STATES PATENTS bustionchamber drops below a value preset in terms 2,046,767 7/1936 Campbell431/174 of the l l spfaed of the gases needed at the 2335.471 l H1943Ashcraft I I 131/31 let of the In ectors In order to prevent a flashback, the 3939522 6/1962 Soho 431/9O injector with the largestcross-section shall be cut out 3,146,821 9/1964 Wuetig 431/115 ofService, the p y of the Combustion Chamber 3,173,483 3/1965 Br d ct al431/90 being then solely carried out by the remaining injector 3,322,1785/1967 Nah-as 431/61 or injectors. 3,376,098 4/1968 Pryor 431/1583.529.915 9 1970 Tsuji Ct a1. 431/284 Clams, 4 Drawlng Flgul'es3.749.546 7/1973 Reed ct a1 431/89 3,779,689 12/1973 Reed ct a1 431/61PATENTED SEP 2x175 sum 1 0r 2 FIG.1

DILUTION BURNER The present invention relates to a dilution burnercomprising in succession at least one ejector fora fuel gas and acomburent gas, a combustion chamber wherein terminates suchinjector, adilution chamber wherein diluting gases are added to the gases leavingthe combustion chamber, and'a terminal element comprising an outletopening through which the gases, both those originating from thecombustion chamber and the dilution gases, enter the furnace whereon ismounted the burner.

By the term dilution burner is to be understood a burner wherein air orwaste gases are added to the gas issuing from the burners.

The conventional dilution burners known up to the present, if they arebased on premixing' the fuel and the comburent, only pennit varying thecalorific flow and the volumetric flow within fairly narrow limits. Thisresults essentially from the risks of flash back to the injector.Indeed, in order to prevent such flash back, it 'is mandatory for therate of the discharge of the gases at the injector to be always higherthan the rate of combustion of the combustion gas comburent gas mixture.One of the principal objects of this invention is to overcome thisdrawback and, moreover, to make avail able a dilution burner permittingthe simultaneous variation of these two flows.

There are also known on the market dilution burners to the flames ofwhich is added air or the gases picked up inside the furnace to beheated by such burners. The mixture of air-formed dilution gases iscarried out in the combustion chamber. Q

This presents inter alia the drawback that the excess quantity of airand, consequentlyfthe dilution thereby is limited by the conditions ofcombustion.

Another reason because of which the known burners only permit achievinga very limited dilution and, consequently, a setting of the temperatureinside the furnace for a specific calorific flow within very narrowlimits is the fact that the mixture of fuel gas and of comburent gas iscarried out within the actual combustion chamber. v

For that purpose, according to the invention, the burner comprises atleast two separate injectors of different injection cross-sections,opening into the entry of the combustion chamber, means being providedin order that, when the injection flow of gas in the combustion chamberdrops below a value preset in terms of the minimum speed of the gasesneeded at the outlet of the injectors in order to prevent a flash back,the injector with the largest cross-section shall be cut out of service,the supply of the combustion chamberbeing then solely carried out by theinjector with. the smallest cross-section.

According to a particular advantageous embodiment of the invention, atleast one delivery pipe for the dilution gases ends in the said dilutionchamber, means being provided to adjust the flow of this dilution gas insuch pipe independently of the kinetic energy of the combustion gasescrossing the-dilution chamber.

A constructional embodiment of a burner according FIG. I shows adiagrammatic elevational sectional view along line Il in FIG. 2.

, FIG. shows a sectional view along line IIII in FIG.

FIG. 3 shows an elevational sectional view of a detail of the burnershown in FIGS. 1 and 2.

FIG. 4 shows a diagrammatic view of an assembly of burners of the typeshown in FIGS. 1 and 2 and of pipes supplying these burners with amixture of fuel gas comburent gas prepared beforehand.

According to the invention, the dilution burner shown in FIGS. 1' and 2comprises in succession two concentric injectors l and 2, a combustionchamber 3 wherein end these injectors, a dilution chamber 4, whereindilution gases are added to the gases leaving the combustion chamber 3,and a terminal element 5 provided with a central outlet opening 6through which the gases originating from the combustion chamber 3 andmixed with the dilution gases enter the furnace, not shown in theFigures, whereon is mounted the burner. The burner comprises mainly anouter casing 7 of prismatic shape and a cylinder 8 inscribed within suchprism and determining externally the combustion chamber 3. Both such thecasing 7 and the cylinder 8 are produced from ceramic material.

In the embodiment shown in FIGS. 1 and 2, the casing 7 is of squareinternal cross-section, so that between such casing and the externalperiphery of the combustion chamber 3 are formed four longitudinal ducts9 of whichone end terminates in the dilution chamber 4. The other end ofthe ducts 9 terminates in an admission chamber 10 for air or anotherdilution gas extending around the injectors l and 2.

The combustion chamber 3 communicates with the dilution chamber 4 bymeans of a throttle terminating in a port 1 1 having a diameter lessthan that of the outlet opening 6 provided in the terminal element 5 andcoaxial with such opening. Accordingly, by selecting thewidth of thedilution chamber 4 and the thickness of the element 5, the unitcomprising the port 11, the opening 6, and the space of the dilutionchamber 4 extending between such port and such opening, provides aninjector by means of which it is possible to establish in such space,thanks to the considerable kinetic energy of the gases escaping from thecombustion chamber 3 through the port 11, a depression sufficient toensure that the dilution gases are drawn by the gases of combustionthrough the outlet opening 6 into the furnace, not shown, whereon ismounted the burner.

If no substantial amount of air or other diluting gas is introduced inthe chamber 4 by the ducts 9, the depression in the chamber 4 alsodraws, through channels 12 passing right through the terminal element ,5and extending. ring-like around the central opening 6, waste gasespositioned inside the furnace. These gases drawn from the furnace areadmixed with the waste gases leaving the combustion chamber 3 and aredrawn by the latter, thanks to their considerable kinetic energy,through the opening 6 back again into the furnace. The burner accordingto the invention is thus, because of this phenomenon, self'diluent underthese conditions.

If air or other diluting gases, such as waste gases are introduced inthe 'dilution' chamber 4 by the ducts 9, such air or other diluting gasis drawn in part by the stream'of waste gases leaving the combustionchamber 3 in order to be thus admitted through the opening 6 inside thefurnace; the otherportion ,of the air or of such dilution gasesleaves'through the channels 12 provided in the terminal element 5 inorder to be drawn by the stream escaping from the opening 6 so as toprovide an additional dilution of the waste gases leaving the combustionchamber 3. Under these conditions-the burner is likewise diluted, andthere exists thus a double dilution.

The temperatureof such stream is determined, for a given flow ofdilution gas, by the flow of the burner.

The dilution burner according to the invention thus ejects at its outletopening 6 provided in the element a stream of gas which is stable,diluted, and towhich is imparted a high velocity, the latter resultingfrom the narrowness of the opening, from the considerable volume ofwaste gases, caused by the. high temperature ruling inside thecombustion chamber, and from the addition of dilution gas.

A mixing-measuring valve, not shown in the FIGS. 1 and 2, to which leada duct for comburent gas and a fuel gas delivery pipe is mountedupstream of each injector l and 2 in such a manner that the latter shallbe supplied with a fuel mixed with a comburent beforehand.

The selection of the volume of comburent gas fuel gas mixture admittedby means of the said valve inside the injectors l and 2 controls thecalorific flow of the burner, while the adjustment of the quantity ofdilution gas, admitted by the channels 9 to the dilution chamber,controls the temperatureand the volume of the gases ejected by theburner through the opening 6. The quantity of dilution gas admitted bythe channels 9 can be adjusted, for example, by a manually controlledvalve 38 located in the pipe 39 leading to the admission chamber 10. Itis also possible, in the case of specific uses, to adjust right away thetemperature of the dilution gases admitted to the dilution chamber 4through the ducts 9.

The limits of adjustment of the calorific output are dependent on thecharacteristics of the injectors, of the maximum feed pressure and ofthe minimum feed pressure of the latter. The upper control limits of thedilution gas flow are set by the feed pressure. As regards the lowerlimits, they are practically nil, as all the components making up theburner are of fire-proof material of suitable quality and able to resistthe release of heat, by conductivity, of the combustion chamber.

In order that the calorific output may drop below the total flow of thefuel gas comburent gas mixture which risks causing the flash back,according to the invention, the burner comprises a double injector ortwo injectors as already mentioned above, in order that the reduced flowmay only be admitted by,-a reduced sec- I tion of injection.

In the embodiment shown in FIG. 1 and illustrated in greater detail inFIG. 3, the two injectors or the double injector comprise two coaxialducts.

The two injectors l and 2 have different injection cross-sections, theinjector 1 having the larger section.

Means are provided in order that, when the injection flow of the gasesinto the combustion chamber drops below a value preset in terms of theminimum speed of the gases needed at the outlet of the injectors toprevent a flash back, the injector 1 with the larger crosssection, shallbe cut out of service, the supply of the combustion chamber being thensolely carried out by i the injector 2 having the smaller injectionsection.

The said means comprise automatic valves 13 and 14 mounted on the feed.pipe 15 of the injector l and on the feed pipe 16 of the injector 2,respectively. The

' l3 and 14 are controlled in terms of the pressure present in the pipes15 and 16 downstream of these valves. This pressure is measured by thegauges l7 and 18 provided in the said pipes 15 and 16 respectively,downstream of the valves 13 and 14.

As soon as the pressure of the fuel gas comburent gas mixture flowing inthe pipe 15 drops below the critical value of the flash back, the valve13 closes and the burner is solely supplied through the injector 2. Thepurpose of the valve 14 will be obvious from the subsequent explanationof the purpose of the valves 26 and 28.

In order to prevent the mixture fuel gas comburent gas, remaining in thepipe 15 between the valve 13 and the outlet of injector 1 from burningand giving rise to a detonation, according, to the invention thecomposition of such mixture flowing in the pipe 15 may be modifiedbefore closing the valve 13 in order that such composition shall falloutside the limits of combustion of the said mixture.

A particular embodiment of the invention making it possible to implementsuch modification of composition is shown in FIG. 4.

In that Figure only ,two burners mounted in parallel are shown, but itis evident that this quantity may vary in accordance with the dimensionsof the furnace.

The injectors 1 and2 are separately fed by means of mixing-measuringvalves 23 and 24 of a type suitable to assure a constant fuel gascomburent gas ratio for all flows.

Theequipment illustrated in FIG. 4 comprises a common pipe 19 for thetwo injectors l and 2 supplying the comburent gas, particularly thecombustion air, to these injectors. A main valve 20, motor driven orotherwise, is provided in this pipe 19, such valve permitting theadjustment of the flow of the burners, mounted in parallel, according tothe calorific output required.

This common pipe 19 is forked downstream of the valve 20 in order toprovide two separate pipes 15 and 16 for the two injectors l and 2. Atubing 21 and a tubing 22 for supplying the fuel gas are led off each ofthese separate pipes 15 and 16 respectively by means of mixing-measuringvalves 23 and 24 respectively for these two gases. Intermediate valves25 and 26 are provided for each of these separate pipes 15 and 16respectively upstream of the mixing-measuring valves 23 and 24.

As in the embodiment shown in FIG. 3, pressure gauges 17 and 18 aremounted in each of these separate pipes 15 and 16 downstream of thevalves 23 and 24. The pressure gauge 17 actuates the valves 25 and 27,and the pressure gauge 18 actuates the valves 26 and 28.

The separate pipes terminate at each one of the injectors 1 and 2 ofeach burner and feed the latter with a mixture of fuel gas and comburentgas in preset quantities.

As soon as the pressure of the fuel gas comburent gas mixture, monitoredby the gauge 17, drops below a pressure slightly above the criticalpressure value causing a flash back, the valve 27 (which may, forexample, be a magnetic valve) is closed by the motor 31, causing adepletion of fuel gas in the feed pipe 15. Subsequently valve 25 isclosed by motor 33, stopping any flow in the pipe 15 supplying theinjector l.

The time lag between the closure of valve 27 and that of valve may becontrolled by a time relay according to the nature of the fuel gas inorder that the gaseous mixture, remaining in the pipe 15, shall not beinflamed. The calorific fiow is then solely supplied by the injector 2.

When an increase of the calorific flow is required, valve 25 opensagain. As soon as the pressure in the pipe 15, monitored by the gauge17, exceeds the critical value, below which there is a risk of a flashback to injector l, valve 27 opens and injector 1 is cut in again forservice. I

Should, however, the calorific flow required be less than thatcorresponding to the minimum velocity of the gases at the outlet of theinjector 2 to prevent a flash back to this injector, a contactor 37actuated by the gauge 18, causes the closure of valve 28 via a motor 32.The main valve 20 or the valve 26 may then be closed by a motor 34without any flash back arising asthe gaseous mixture has beensufficiently depletedin the pipe 16.

From the description hereinbefore given of a particular embodiment of aburner according to the invention, it follows that the range of dilutionby means of air or another gas, such as a waste gas, is practicallyunlimited. When no further injection of dilution gas originating fromthe ducts 9 takes place, the dilution by the fumes of the furnace drawninto the chamber continues.

An ejection of gas at reduced temperature and of large flow is thusalways assured at the burner outlet. This establishes in the furnacechamber a very extensive isotherm because of the entraining action ofthe furnace atmosphere from the high speed of the gas stream on leavingthe burner.

The entraining action of the furnace atmosphere may be advantageouslyenhanced by an appropriate lay-out of the burners within the enclosureto be heated, for example by providing a substantially tangentialinjection.

From the description hereinbefore given, it also follows that thethermal output of the burner according to the invention may be adjustedwithin very wide limits without risk of a flash back. It is, indeed,possible to select the outlet cross-sections of the injectors 1 and 2taking the rate of combustion of the fuel gas used into account.

Thanks to the use of mixing-measuring valves of a suitable type, theratio of the comburent gas fuel gas mixture supplied to all the burnersof the same run remains constant when the flow of the run consideredvaries.

It is well understood that the invention is not limited to the describedembodiments, but that many modified versions may be contemplated withoutdeparting from the scope of the present patent application.

Thus, two, quite separate, non-concentric injectors might, if need be,be provided or even more than two of such injectors with differentinjection cross-sections.

What I claim is:

l. A dilution burner comprising:

a. a combustion chamber;

b. a first injector opening into the entry of said combustion chamber; I

c. a first feed pipe leading to said first injector;

d. a second injector, separate from: and independent of said firstinjector and having a larger crosssection than said first injector,likewise opening into the entry of said combustion chamber;

e. a second feed pipe leadjng to said second injector;

f. first means for cutting said second injector out of service when theinjection flow of gas in said combustion chamber drops below a value.preset in terms of the minimum speed of the gases needed at the outlets,of said injectors to prevent a flash back, said first means beinglocated in said second feed pipe, said first means comprising anautomatic valve mounted in said second feed pipe, the opening andclosing of said automatic valve being controlled in terms of thepressure ruling in said feed pipe downstream of said automatic valve;

g. second means for modifying the composition of the fuel-comburentmixture i. in the portion of said second feed pipe located between saidsaid second injector and said first means,

' -ii. to a composition which is' outside the limits of combustion ofthe mixture, and iii. prior to the operation of said first means;

h. a common pipe for supplying comburent gas to both said first injectorand said second injector;

i. a main'valve mounted in said common pipe;

j. a first separate pipe leading from said common pipe downstream ofsaid main valve to provide comburent gas to said first feed pipe;

k. a second separate pipe leading from said common v pipe downstream ofsaid main valve to provide comburent gas to said second feed pipe, saidfirst means being located in said second separate pipe;

1. a first mixing-measuring valve for the comburent and fuel gasesentering said first feed pipe;

m. a second mixing-measuring valve for the comburent and fuel gasesentering said second feed pipe;

n. a first intermediate valve located in said first separate pipe;

0. a first fuel supply pipe leading to said first mixingmeasuring valve;1

p. a second fuel supply pipe leading to said second mixing-measuringvalve, said second means being located in said second fuel pipe;

q. a second intermediate valve located in said first fuel supply pipe;and

r. means for controlling said valves with reference to the pressureruling in said first and second separate pipes downstream of said firstand second mixingmeasuring valves such that: I i. when the pressure ineither of said first and second separate pipes drops below a valuepreset in terms of the minimum speed of the gases needed at the outletsof said injectors to prevent a flashback, said second means is firstcaused to close and, subsequently, said first means is caused to close,the time lag occurring between these two closures being calculated sothat the composition of the fuel-comburent mixture remaining in saidsecond feed pipe and the portion of said second separate pipe downstreamof said second mixingmeasuring valve is sufficiently depleted of fuel,prior to cutting said second injector out of service, to prevent theburning of the mixture; ii. when subsequently an increase of the flow isagain required, said first means is first opened and, subsequently, saidsecond means is opened when the pressure ruling in said second separatepipe exceeds said preset value, and' iii. when said second injector hasbeen cut out of service and the velocityof the gases at the outlet ofsaid first injector is less than the velocity required to prevent aflashback, said second intermediate valve is first closed and,subsequently, said first intermediate valve is also closed,'thus cuttingsaid first injector out of service.

2. A dilution burner comprising:

a. a combustion chamber;

b. a first injector opening into the entry of said combustion chamber; 1

c. a first feed pipe leading to said first injector;

d. a second injector having a larger cross-section than and beingcoaxial to said first injector, said second injector being separate fromand independent of said first injector but likewise opening into theentry of said combustion chamber;

e. a second feed pipe leading to said second injector;

f. first means for cutting said second injector out of service when theinjection flow of gas in said combustion chamber drops below .a valuepreset in terms of the minimum speed of the gases needed at the outletsof said injectors to prevent a flashback, said first means being locatedin said secondifeed p p g. second means for modifying thecomposition ofthe fuel-comburent mixture i. inthe portion of said second feed pipelocated between said second injector and said first means,

ii. to a composition which is outside the limits of combustion of themixture, and

iii. prior to the operation of said first means;

h. a common pipe for supplying comburent gas to both said first injectorand said second injector;

i. a main valve mounted in said common pipe;

j. a first separate pipe leading from said commonpipe downstream of saidmain valve to providecomburent gas to said first feed pipe;

k. a second separate pipe leading from said common pipe downstream ofsaid main valve to provide comburent gas to said second feed pipe, saidfirst means being located in said second separate pipe;

l. a first mixing-measuring valve for the comburent and fuel gasesentering said first feed pipe;

m. a second mixing-measuring valve for the comburent and fuel gasesentering said second feed pipe;

n. a first intermediate valve located in said first separate pipe;

o. a first fuel supply pipe leading to said first mixingmeasuring valve;

p. a second fuel supply pipe' leading to said second mixing-measuringvalve, said second means being located in said second fuel supply pipe;

q. a second intermediate valve located in said first fuel supply pipe;and i i r. means for controlling said valves with reference to thepressure ruling in said first and second separate pipes downstream ofsaid first and second mixingmeasuring valves such that:

i. when the pressure in either of said first and Second separate pipesdrops below a value preset in terms of the minimum speed of the gasesneeded at the outlets of said injectors to prevent a flashback, saidsecond means is first caused to close 8 land, subseque'ntly, said firstmeans is caused to closejthe tirne lag occurring between these twoclosures being calculated so that the composition of the fuel-comburentmixture remaining in said second feed pipeand the portion of'said secondseparate pipe downstream of said second mixingmeasuring valve issufficiently depleted of fuel, 'prior to cutting said second injectorout of service, to prevent the burning of the mixture;

ii. when subsequently an increase of the flow is again required,said'first means is first opened and, subsequently, said second means isopened when the pressure ruling in said second separate pipe-exceedssaid preset value, and

'iii. when said second injector has been cut out of service and thevelocity of the gases at the outlet of said first injector is less thanthe velocity required to prevent afiashback, said second intermediatevalve is first closed, and subsequently, .said first intermediate valveis also closed, thus cutting said first injector out of service.

3.- A dilution burner comprising:

a. a-* combustion chamber;

b. a first injector for a mixture of a fuel gas and a comburent gas,said first injector opening into the entry of said combustion chamber;

c. a second injector having a smaller cross-section than and beingcoaxial to said first injector, said second injector likewise openinginto the entry of said combustion chamber but said second injector beingseparate from. and independent of said first injector,

d. means for cutting said first injector out of service when theinjection flow of gas in said combustion chamber drops below a valuepreset in terms of the minimum speed of the gases needed at the outletsof said first and second injectors inorder to prevent a flashback; v

e. a feed pipe leading to said first injector, said means being mountedin said feed pipe;

f. a pressure gauge mounted in said feed pipe between said means andsaid first injector; and

g. means for closing said means when said pressure gauge senses a valuewhich indicates that the injection flow of gas in said combustionchamber has dropped below the preset value referred to in subparagraph(d).

4. A dilution burner comprising:

a. a combustion chamber;

b. a first injector for a mixture of a fuel gas and a comburent gas,said first injector opening into the entry of said combustion chamber;

c. a second injector for a mixture of a fuel gas and a comburent gas,said second injector likewise opening into the entry of said combustionchamber but said second injector beingseparat'e from and independent ofsaid first injector;

d. first meansfor cutting said first injector out of service when theinjection flow of gas in said combustion chamber drops below a valuepreset in terms of minimum speed of the gases needed at the outlets ofsaid first and second injectors in order to prevent a flashback;

e. second means for modifying the composition of the fuel-comburent rrlixturein said first injector so that the composition isoutsijde thelimits of combustion of the mixture;

f. third means for modifying the composition of the fuel-comburentmixture in said second injector so that the composition is outside thelimits of combustion of the mixture;

g. a feed pipe leading to said first injector, said first means beingmounted in said feed pipe;

h. a pressure gauge mounted in said feed pipe between said first meansand said first injector; and

i. means for closing said first means when said pressure gauge senses avalue which indicates that the injection flow of gas in said combustionchamber has dropped below the preset value referred to in subparagraph((1).

5. A burner comprising at least two separate independent injectors ofdifferent injection cross-sections opening to one another mounted intothe entry of a same combustion chamber and a mixing measuring valve towhich lead a feed duct for cumburent gas and a fuel gas delivery pipebeing mounted upstream of each injector in such a manner that eachinjector is supplied with a fuel mixed with a comburent beforehand,means being provided, so that, when regulating the gas output theinjection speed decreases, just before the injection speed of gas intothe combustion chamber drops below a value preset in terms of theminimum speed of the gas needed at the outlet in the injectors in orderto prevent a flashback, the injector with the larger or largestcross-section is automatically cut out of service, the supply of thecombustion chamber being then solely carried out by the remaininginjector or injectors, said means comprising automatic valves mounted infeed pipes leading to each of the injectors, the opening and closure ofwhich are controlled in terms of the pressure ruling in the pipe,downstream of such valves.

6. A burner comprising at least two separate independent injectors ofdifferent injection cross-sections opening to one another mounted intothe entry of a same combustion chamber and a mixing measuring valve towhich lead a feed duct for cumburent gas and a fuel gas delivery pipebeing mounted upstream of each injector in such a manner that eachinjector is supplied with a fuel mixed with a comburentbeforehand, meansbeing provided, so that, when regulating the gas output the injectionspeed decreases, just before the injection speed of gas into thecombustion chamber drops below a value preset in terms of the minimumspeed of the gas needed at the outlet in the injectors in.

order to prevent a flashback, the injector with the larger or largestcross-section is automatically cutout of service, the supply of thecombustion chamber being then solely carried out by the remaininginjector or injectors, and further comprising a dilution chamber whereindiluting gases are added to the gases leaving the combustion chamber anda terminal element comprising an outlet opening through which the gases,originating from the combustion chamber admixed with the dilution gases,are expelled, characterized in that at least one delivery pipe for thedilution gases ends in the said dilution chamber, means being providedto adjust the flow of this dilution gas in such pipe independently ofthe kinetic energy of the combustion gases crossing the dilutionchamber.

7. A dilution burner comprising:

a. a combustion chamber;

b a first injector opening into the entry of said combustion chamber;

c. a first feed pipe leading to said first injector;

d. a'second injector, separate from and independent of said firstinjector and having a larger crosssection than said first injector,likewise opening into the entry of said combustion chamber;

e. a second feed pipe leading to said second injector;

f. first means for cutting said second injector out of service when theinjection flow of gas in said combustion chamber drops below a valuepreset in terms of the minimum speed of the gases needed at the outletsof said injectors to prevent a flashback, said first means being locatedin said second feed p p g. second means for modifying the composition ofthe fuel-comburent mixture i. in the portion of said second feed pipelocated between said said second injector and said first means,

ii. to a composition which is outside the limits of combustion of themixture, and

iii. prior to the operation of said first means;

h. a common pipe for supplying comburent gas to both said first injectorand said second injector;

i. a main valve mounted in said common pipe;

j. a first separate pipe leading from said common pipe downstream ofsaid main valve to provide comburent gas to said first feed pipe;

k. a second separate pipe leading from said common pipe downstream ofsaid main valve to provide comburent gas to said second feed pipe, saidfirst means being located in said second separate pipe;

1. a first mixing-measuring valve for the comburent and fuel gasesentering said first feed pipe;

in. a second mixing-measuring valve for the comburent and fuel gasesentering said second feed pipe;

n. a first intermediate valve located in said first separate pipe;

0. a first fuel supply pipe leading to said first mixingmeasuring valve;

p. a second fuel supply pipe leading to said second mixing-measuringvalve, said second means being located in said second fuel pipe;

q. a second intermediate valve located in said first fuel supply pipe;and

r. means for controlling said valves with reference to the pressureruling in said first and second separate pipes downstream of said firstand second mixingmeasuring valves such that:

i. when'the pressure in either of said first and second separate pipesdrops below a value preset in terms of the minimum speed of the gasesneeded at the outlets of said injectors to prevent a flashback, saidsecond means is first caused to close and, subsequently, said firstmeans is caused to close, the time lag occurring between these twoclosures being calculated so that the composition of the fuelcomburentmixture remaining in said second feed pipe and the portion of saidsecond separatepipe downstream of said second mixing-, measuring valveis sufficiently depleted of fuel, prior to cutting said second injectorout of service, to prevent the burning of the mixture;

ii. when subsequently an increase of the flow is iii. when said secondinjector has been cut out of service and the velocity of the gases atthe outlet of said first injector is less than the velocity required toprevent a flashback, said second intermediate valve is firs't closedand, subsequently, said first intermediatevalve is also closed, thuscutting said first injector out'of service.

8. A dilution burner comprising:

a. a combustion chamber;

b. a first injector opening into the entry of said combustion chamber;

c. a first feed pipe leading to said first injector;

d. a second injector having a larger cross-section than and beingcoaxial to said first injector, said second injector being separate fromand independent of said first injector but likewise opening into theentry of said combustion chamber;

e. a second feed pipe leading to said second injector;

f. first means for cutting said second injector out of service when theinjection flow of gas in said combustion chamber drops below a valuepreset in terms of the minimum speed of the gases needed at the outletsof said injectors to prevent a flashback, said first means being locatedin said second feed P p g. a common pipe for supplying comburent gas toboth said first injector and said second injector;

h. a main valve mounted in said common pipe;

i. a first separate pipe leading from said common pipe downstream ofsaid main valve to provide comburent gas to said first feed pipe;

j. a second separate pipe leading from said common pipe downstream ofsaid main valve to provide comburent gas to said second feed pipe, saidfirst means being located in said second separate pipe;

k. a first mixing-measuring valve for the comburent and fuel gasesentering said first feed pipe;

I. a second mixing-measuring valve for the comburent and fuel gasesentering said second feed pipe;

m. a first intermediate valve located in said first separate pipe;

n. a first fuel supply pipe leading to said first mixingmeasuring valve;

0. a second fuel supply pipe leading to said'second mixing-measuringvalve;

p. a second intermediate valve located in said first fuel supply pipe;

q. a third intermediate valve iocated in said second fuel supply pipe;and

r. means for controlling said valves with reference to the pressureruling in said first and second separate pipes downstream of said firstand second mixingmeasuring valves such that:

i. when the pressure in either of said first and second separate pipesdrops below a value preset in terms of the minimum speed of the gasesneeded at the outlets of said injectors to prevent a flashback, saidthird intermediate valve is first caused to close and, subsequently,said first means is caused to close, the time lag occurring betweenthese two closures being calculated so that the composition of thefuel-comburent mixture remaining in said second feed pipe and theportion of said second separate pipe downstream of said secondmixing-measuring valve is sufficiently de pleted of fuel, prior tocutting said second injector out of service, to prevent the burning ofthe mixture;

ii. when subsequently an increase of the flow is again required, saidfirst means is first opened and, subsequently, said third intermediatevalve is opened when the pressure ruling in said second separate pipeexceeds said preset value, and

iii. when said second injector has been cut out of service and thevelocity of the gases at the outlet of said first injector is less thanthe velocity required to prevent a flashback, said second intermediatevalve is'first closed and, subsequently, said said first intermediatevalve is also closed,

thus cutting said first injector out of service.

9. A dilution burner as claimed in claim 5, characterized in that meansare provided in order to modify in the portion of the feed pipe to theinjector having the largest cross-section located between the latter andthe said automatic valve; the composition of the fuelcornburent mixturein order that it shall be outside the limits of combustion of suchmixture prior to such injector being cut out of service.

jectors, a pipe for the fuel gas supply being led off each of theseseparate pipes by means of a mixing-measuring valve of the comburent andfuel gases, while an intermediate valve is provided for each of theseseparate pipes as well as in each of the pipes ledoff the latterupstream of the mixing-measuring valves, these separates pipesterminating each at one of the injectors, means for controlling saiddifferent valves with reference to the pressure ruling in the separatepipes downstream of such mixing-measuring valves in such a manner that,when the pressure drops, in one of these separate pipes, below a valuepreset in terms of the minimum speed of the gases needed at the outletof the injectors in order to prevent the flash back, the intermediatevalve mounted in the fuel gas supply pipe led off the separate pipesupplying the-injector of larger cross-section is caused to close, andsubsequently also the intermediate valve, mounted in the separate pipesupplying the latter, the time lag occurring between these two closuresbeing calculated in such a manner that the composition of thefuel-comburent mixture remaining in the portion of the separate injectorof large cross-section downstream of the mixing-measuring valve mountedin the latter, shall be sufficiently depleted of fuel, prior to cuttingthe latter injector out of service, to prevent the burning of suchmixture and so that, when subsequently an increase of the flow is againrequired, the intermediate valve mounted in the separate pipe supplyingthe large cross-section injector shall be opened and that subsequentlytakes place the opening of the corresponding intermediate valve mountedin the fuel gas feed pipe at the time when the pressure ruling in suchseparate pipe shall exceed the said minimum value, and lastly, in such amanner that, when the large section injector has been cut out of serviceand the velocity of the gases at the outlet of the injector of smallercrosssection shall be less than the velocity required to prevent theflash back, the intermediate valve mounted in the fuel gas supply pipeled off the supply pipe of the injector with the smaller cross-sectionshall be first closed and subsequently the said main valve and/or theremaining intermediary valve shall also be closed, thus cutting theinjector of smaller cross-section also out of service.

11. A dilution burner according to claim 6, characterized in that thecombustion chamber communicates with the dilution chamber by means of athrottle terminating in a port having a diameter less than that of thesaid outlet opening provided in the terminal element and coaxial withsuch opening, so that the unit comprising such throttle, such openingand the space of the dilution chamber extending between the port and theopening provides an injector permitting to establish in such space,thanks to the considerable kinetic energy of the combustion gasesescaping from the combustion chamber through such port, a depressionsufiicient -to ensure that the said dilution gases shall be drawn by thegases of combustion through the said outlet opening into the furnacewhereon is mounted the burner.

12. A dilution burner according to claim 11, characterized in that itcomprises several dilution gas supply ducts terminating in the dilutionchamber in locations uniformly distributed around the said port throughwhich the combustion gases are admitted in such chamber.

13. A dilution according to claim 11, characterized in that the centresof the said port and opening are located in the longitudinal axis of theburner.

14. A dilution burner according to claim 6, characterized in that thesaid terminal eler'nent comprises at least one channel passing rightthrough such element, permitting the furnace to be heated to communicatewith the dilution chamber, thus making it possible to draw through suchchannel gases from the furnace into the dilution chamber, such gasesthus providing dilution gases for the gases originating from thecombustion chamber,

15. A dilution burner according to claim 9, characterized in that themeans provided to modify in the portion of the feed pipe to the injectorhaving the largest cross-section the composition of the fuel-comburentmixture, prior to cutting such injector out of service, comprise a pipecommon for both injectors supplying the comburent gas for the latter andwherein is mounted a main valve, such common pipe being forkeddownstream of such valve in order to provide separate pipes for the twoinjectors, a pipe for the fuel gas supply being led off each of theseseparate pipes by means of a mixing-measuring valve of the comburent andfuel gases, while an intermediate valve is provided for each of theseseparate pipes as well as in each of the pipes led off the latterupstream of the mixing-measuring valves, these separate pipesterminating each at one of the injectors, means for controlling saiddifferent valves with reference to the pressure ruling in the separatepipes downstream of such mixing-measuring valves in such a manner that,when the pressure drops, in one of these separate pipes, below a valuepreset in terms of the minimum speed of the gases needed at the outletof the injectors in order to prevent a flash back, the intermediatevalve mounted in the fuel gas supply pipe led off the separate pipesupplying the injector of larger cross-section is caused to close, andsubsequently also the intermediate valve, mounted in the separate pipesupplying the latter, the time lag occurring between these two closuresbeing calculated in such a manner that the composition of thefuel-comburent mixture remaining in the portion of the separate injectorof large cross-section downstream of the mixing-measuring valve mountedin the latter, shall be sufficiently depleted of fuel, prior to cuttingthe latter injector out of service, to prevent the burning of suchmixture and so that, when subsequently an increase of the flow is againrequired, the intermediate valve mounted in the separate pipe supplyingthe large cross-section injector shall be opened and that subsequentlytakes place the opening of the corresponding intermediate valve mountedin the fuel gas feed pipe at the time when the pressure ruling in suchseparate pipe shall exceed the said minimum value, and lastly, in such amanner that, when the large section injector has been cut out of serviceand the velocity of the gases at the outlet of the injector of smallercross-section shall be less than the velocity required to prevent theflash back, the intermediate valve mounted in the fuel gas supply pipeled off the supply pipe of the injector with the smaller cross-sectionshall be first closed and subsequently the said main valve and/or theremaining intermediary valve shall also be closed, thus cutting theinjector of smaller cross-section also out of service.

1. A dilution burner comprising: a. a combustion chamber; b. a firstinjector opening into the entry of said combustion chamber; c. a firstfeed pipe leading to said first injector; d. a second injector, separatefrom and independent of said first injector and having a largercross-section than said first injector, likewise opening into the entryof said combustion chamber; e. a second feed pipe leadjng to said secondinjector; f. first means for cutting said second injector out of servicewhen the injection flow of gas in said combustion chamber drops below avalue preset in terms of the minimum speed of the gases needed at theoutlets of said injectors to prevent a flash back, said first meansbeing located in said second feed pipe, said first means comprising anautomatic valve mounted in said second feed pipe, the opening andclosing of said automatic valve being controlled in terms of thepressure ruling in said feed pipe downstream of said automatic valve; g.second means for modifying the composition of the fuel-comburent mixturei. in the portion of said second feed pipe located between said saidsecond injector and said first means, ii. to a composition which isoutside the limits of combustion of the mixture, and iii. prior to theoperation of said first means; h. a common pipe for supplying comburentgas to both said first injector and said second injector; i. a mainvalve mounted in said common pipe; j. a first separate pipe leading fromsaid common pipe downstream of said main valve to provide comburent gasto said first feed pipe; k. a second separate pipe leading from saidcommon pipe downstream of said main valve to provide comburent gas tosaid second feed pipe, said first means being located in said secondseparate pipe; l. a first mixing-measuring valve for the comburent andfuel gases entering said first feed pipe; m. a second mixing-measuringvalve for the comburent and fuel gases entering said second feed Pipe;n. a first intermediate valve located in said first separate pipe; o. afirst fuel supply pipe leading to said first mixing-measuring valve; p.a second fuel supply pipe leading to said second mixing-measuring valve,said second means being located in said second fuel pipe; q. a secondintermediate valve located in said first fuel supply pipe; and r. meansfor controlling said valves with reference to the pressure ruling insaid first and second separate pipes downstream of said first and secondmixing-measuring valves such that: i. when the pressure in either ofsaid first and second separate pipes drops below a value preset in termsof the minimum speed of the gases needed at the outlets of saidinjectors to prevent a flashback, said second means is first caused toclose and, subsequently, said first means is caused to close, the timelag occurring between these two closures being calculated so that thecomposition of the fuel-comburent mixture remaining in said second feedpipe and the portion of said second separate pipe downstream of saidsecond mixing-measuring valve is sufficiently depleted of fuel, prior tocutting said second injector out of service, to prevent the burning ofthe mixture; ii. when subsequently an increase of the flow is againrequired, said first means is first opened and, subsequently, saidsecond means is opened when the pressure ruling in said second separatepipe exceeds said preset value, and iii. when said second injector hasbeen cut out of service and the velocity of the gases at the outlet ofsaid first injector is less than the velocity required to prevent aflashback, said second intermediate valve is first closed and,subsequently, said first intermediate valve is also closed, thus cuttingsaid first injector out of service.
 2. A dilution burner comprising: a.a combustion chamber; b. a first injector opening into the entry of saidcombustion chamber; c. a first feed pipe leading to said first injector;d. a second injector having a larger cross-section than and beingcoaxial to said first injector, said second injector being separate fromand independent of said first injector but likewise opening into theentry of said combustion chamber; e. a second feed pipe leading to saidsecond injector; f. first means for cutting said second injector out ofservice when the injection flow of gas in said combustion chamber dropsbelow a value preset in terms of the minimum speed of the gases neededat the outlets of said injectors to prevent a flashback, said firstmeans being located in said second feed pipe; g. second means formodifying the composition of the fuel-comburent mixture i. in theportion of said second feed pipe located between said second injectorand said first means, ii. to a composition which is outside the limitsof combustion of the mixture, and iii. prior to the operation of saidfirst means; h. a common pipe for supplying comburent gas to both saidfirst injector and said second injector; i. a main valve mounted in saidcommon pipe; j. a first separate pipe leading from said common pipedownstream of said main valve to provide comburent gas to said firstfeed pipe; k. a second separate pipe leading from said common pipedownstream of said main valve to provide comburent gas to said secondfeed pipe, said first means being located in said second separate pipe;l. a first mixing-measuring valve for the comburent and fuel gasesentering said first feed pipe; m. a second mixing-measuring valve forthe comburent and fuel gases entering said second feed pipe; n. a firstintermediate valve located in said first separate pipe; o. a first fuelsupply pipe leading to said first mixing-measuring valve; p. a secondfuel supply pipe leading to said second mixing-measuring valve, saidsecond means being located in said second fuel supply pipe; q. a secondintermediate valve located in said fiRst fuel supply pipe; and r. meansfor controlling said valves with reference to the pressure ruling insaid first and second separate pipes downstream of said first and secondmixing-measuring valves such that: i. when the pressure in either ofsaid first and second separate pipes drops below a value preset in termsof the minimum speed of the gases needed at the outlets of saidinjectors to prevent a flashback, said second means is first caused toclose and, subsequently, said first means is caused to close, the timelag occurring between these two closures being calculated so that thecomposition of the fuel-comburent mixture remaining in said second feedpipe and the portion of said second separate pipe downstream of saidsecond mixing-measuring valve is sufficiently depleted of fuel, prior tocutting said second injector out of service, to prevent the burning ofthe mixture; ii. when subsequently an increase of the flow is againrequired, said first means is first opened and, subsequently, saidsecond means is opened when the pressure ruling in said second separatepipe exceeds said preset value, and iii. when said second injector hasbeen cut out of service and the velocity of the gases at the outlet ofsaid first injector is less than the velocity required to prevent aflashback, said second intermediate valve is first closed, andsubsequently, said first intermediate valve is also closed, thus cuttingsaid first injector out of service.
 3. A dilution burner comprising: a.a combustion chamber; b. a first injector for a mixture of a fuel gasand a comburent gas, said first injector opening into the entry of saidcombustion chamber; c. a second injector having a smaller cross-sectionthan and being coaxial to said first injector, said second injectorlikewise opening into the entry of said combustion chamber but saidsecond injector being separate from and independent of said firstinjector, d. means for cutting said first injector out of service whenthe injection flow of gas in said combustion chamber drops below a valuepreset in terms of the minimum speed of the gases needed at the outletsof said first and second injectors in order to prevent a flashback; e. afeed pipe leading to said first injector, said means being mounted insaid feed pipe; f. a pressure gauge mounted in said feed pipe betweensaid means and said first injector; and g. means for closing said meanswhen said pressure gauge senses a value which indicates that theinjection flow of gas in said combustion chamber has dropped below thepreset value referred to in subparagraph (d).
 4. A dilution burnercomprising: a. a combustion chamber; b. a first injector for a mixtureof a fuel gas and a comburent gas, said first injector opening into theentry of said combustion chamber; c. a second injector for a mixture ofa fuel gas and a comburent gas, said second injector likewise openinginto the entry of said combustion chamber but said second injector beingseparate from and independent of said first injector; d. first means forcutting said first injector out of service when the injection flow ofgas in said combustion chamber drops below a value preset in terms ofminimum speed of the gases needed at the outlets of said first andsecond injectors in order to prevent a flashback; e. second means formodifying the composition of the fuel-comburent mixture in said firstinjector so that the composition is outside the limits of combustion ofthe mixture; f. third means for modifying the composition of thefuel-comburent mixture in said second injector so that the compositionis outside the limits of combustion of the mixture; g. a feed pipeleading to said first injector, said first means being mounted in saidfeed pipe; h. a pressure gauge mounted in said feed pipe between saidfirst means and said first injector; and i. means for closing said firstmeans when said pressure gauge senses a valuE which indicates that theinjection flow of gas in said combustion chamber has dropped below thepreset value referred to in subparagraph (d).
 5. A burner comprising atleast two separate independent injectors of different injectioncross-sections opening to one another mounted into the entry of a samecombustion chamber and a mixing measuring valve to which lead a feedduct for cumburent gas and a fuel gas delivery pipe being mountedupstream of each injector in such a manner that each injector issupplied with a fuel mixed with a comburent beforehand, means beingprovided, so that, when regulating the gas output the injection speeddecreases, just before the injection speed of gas into the combustionchamber drops below a value preset in terms of the minimum speed of thegas needed at the outlet in the injectors in order to prevent aflashback, the injector with the larger or largest cross-section isautomatically cut out of service, the supply of the combustion chamberbeing then solely carried out by the remaining injector or injectors,said means comprising automatic valves mounted in feed pipes leading toeach of the injectors, the opening and closure of which are controlledin terms of the pressure ruling in the pipe, downstream of such valves.6. A burner comprising at least two separate independent injectors ofdifferent injection cross-sections opening to one another mounted intothe entry of a same combustion chamber and a mixing measuring valve towhich lead a feed duct for cumburent gas and a fuel gas delivery pipebeing mounted upstream of each injector in such a manner that eachinjector is supplied with a fuel mixed with a comburent beforehand,means being provided, so that, when regulating the gas output theinjection speed decreases, just before the injection speed of gas intothe combustion chamber drops below a value preset in terms of theminimum speed of the gas needed at the outlet in the injectors in orderto prevent a flashback, the injector with the larger or largestcross-section is automatically cut out of service, the supply of thecombustion chamber being then solely carried out by the remaininginjector or injectors, and further comprising a dilution chamber whereindiluting gases are added to the gases leaving the combustion chamber anda terminal element comprising an outlet opening through which the gases,originating from the combustion chamber admixed with the dilution gases,are expelled, characterized in that at least one delivery pipe for thedilution gases ends in the said dilution chamber, means being providedto adjust the flow of this dilution gas in such pipe independently ofthe kinetic energy of the combustion gases crossing the dilutionchamber.
 7. A dilution burner comprising: a. a combustion chamber; b. afirst injector opening into the entry of said combustion chamber; c. afirst feed pipe leading to said first injector; d. a second injector,separate from and independent of said first injector and having a largercross-section than said first injector, likewise opening into the entryof said combustion chamber; e. a second feed pipe leading to said secondinjector; f. first means for cutting said second injector out of servicewhen the injection flow of gas in said combustion chamber drops below avalue preset in terms of the minimum speed of the gases needed at theoutlets of said injectors to prevent a flashback, said first means beinglocated in said second feed pipe; g. second means for modifying thecomposition of the fuel-comburent mixture i. in the portion of saidsecond feed pipe located between said said second injector and saidfirst means, ii. to a composition which is outside the limits ofcombustion of the mixture, and iii. prior to the operation of said firstmeans; h. a common pipe for supplying comburent gas to both said firstinjector and said second injector; i. a main valve mounted in saidcommon pipe; j. a first separate pipe leadiNg from said common pipedownstream of said main valve to provide comburent gas to said firstfeed pipe; k. a second separate pipe leading from said common pipedownstream of said main valve to provide comburent gas to said secondfeed pipe, said first means being located in said second separate pipe;l. a first mixing-measuring valve for the comburent and fuel gasesentering said first feed pipe; m. a second mixing-measuring valve forthe comburent and fuel gases entering said second feed pipe; n. a firstintermediate valve located in said first separate pipe; o. a first fuelsupply pipe leading to said first mixing-measuring valve; p. a secondfuel supply pipe leading to said second mixing-measuring valve, saidsecond means being located in said second fuel pipe; q. a secondintermediate valve located in said first fuel supply pipe; and r. meansfor controlling said valves with reference to the pressure ruling insaid first and second separate pipes downstream of said first and secondmixing-measuring valves such that: i. when the pressure in either ofsaid first and second separate pipes drops below a value preset in termsof the minimum speed of the gases needed at the outlets of saidinjectors to prevent a flashback, said second means is first caused toclose and, subsequently, said first means is caused to close, the timelag occurring between these two closures being calculated so that thecomposition of the fuel-comburent mixture remaining in said second feedpipe and the portion of said second separate pipe downstream of saidsecond mixing-measuring valve is sufficiently depleted of fuel, prior tocutting said second injector out of service, to prevent the burning ofthe mixture; ii. when subsequently an increase of the flow is againrequired, said first means is first opened, and, subsequently, saidsecond means is opened when the pressure ruling in said second separatepipe exceeds said preset value, and iii. when said second injector hasbeen cut out of service and the velocity of the gases at the outlet ofsaid first injector is less than the velocity required to prevent aflashback, said second intermediate valve is first closed and,subsequently, said first intermediate valve is also closed, thus cuttingsaid first injector out of service.
 8. A dilution burner comprising: a.a combustion chamber; b. a first injector opening into the entry of saidcombustion chamber; c. a first feed pipe leading to said first injector;d. a second injector having a larger cross-section than and beingcoaxial to said first injector, said second injector being separate fromand independent of said first injector but likewise opening into theentry of said combustion chamber; e. a second feed pipe leading to saidsecond injector; f. first means for cutting said second injector out ofservice when the injection flow of gas in said combustion chamber dropsbelow a value preset in terms of the minimum speed of the gases neededat the outlets of said injectors to prevent a flashback, said firstmeans being located in said second feed pipe; g. a common pipe forsupplying comburent gas to both said first injector and said secondinjector; h. a main valve mounted in said common pipe; i. a firstseparate pipe leading from said common pipe downstream of said mainvalve to provide comburent gas to said first feed pipe; j. a secondseparate pipe leading from said common pipe downstream of said mainvalve to provide comburent gas to said second feed pipe, said firstmeans being located in said second separate pipe; k. a firstmixing-measuring valve for the comburent and fuel gases entering saidfirst feed pipe; l. a second mixing-measuring valve for the comburentand fuel gases entering said second feed pipe; m. a first intermediatevalve located in said first separate pipe; n. a first fuel supply pipeleading to said first mixing-measuring valve; o. a second fuEl supplypipe leading to said second mixing-measuring valve; p. a secondintermediate valve located in said first fuel supply pipe; q. a thirdintermediate valve located in said second fuel supply pipe; and r. meansfor controlling said valves with reference to the pressure ruling insaid first and second separate pipes downstream of said first and secondmixing-measuring valves such that: i. when the pressure in either ofsaid first and second separate pipes drops below a value preset in termsof the minimum speed of the gases needed at the outlets of saidinjectors to prevent a flashback, said third intermediate valve is firstcaused to close and, subsequently, said first means is caused to close,the time lag occurring between these two closures being calculated sothat the composition of the fuel-comburent mixture remaining in saidsecond feed pipe and the portion of said second separate pipe downstreamof said second mixing-measuring valve is sufficiently depleted of fuel,prior to cutting said second injector out of service, to prevent theburning of the mixture; ii. when subsequently an increase of the flow isagain required, said first means is first opened and, subsequently, saidthird intermediate valve is opened when the pressure ruling in saidsecond separate pipe exceeds said preset value, and iii. when saidsecond injector has been cut out of service and the velocity of thegases at the outlet of said first injector is less than the velocityrequired to prevent a flashback, said second intermediate valve is firstclosed and, subsequently, said said first intermediate valve is alsoclosed, thus cutting said first injector out of service.
 9. A dilutionburner as claimed in claim 5, characterized in that means are providedin order to modify in the portion of the feed pipe to the injectorhaving the largest cross-section located between the latter and the saidautomatic valve, the composition of the fuel-comburent mixture in orderthat it shall be outside the limits of combustion of such mixture priorto such injector being cut out of service.
 10. A dilution burneraccording to claim 9, characterized in that the means to modify in theportion of the injector feed pipe with the largest cross-section thecomposition of the fuel-comburent mixture, prior to cutting suchinjector out of service, comprise a pipe common for both injectorssupplying the comburent gas for the latter and wherein is mounted a mainvalve, such common pipe being forked downstream of such valve in orderto provide separate pipes for the two injectors, a pipe for the fuel gassupply being led off each of these separate pipes by means of amixing-measuring valve of the comburent and fuel gases, while anintermediate valve is provided for each of these separate pipes as wellas in each of the pipes led off the latter upstream of themixing-measuring valves, these separates pipes terminating each at oneof the injectors, means for controlling said different valves withreference to the pressure ruling in the separate pipes downstream ofsuch mixing-measuring valves in such a manner that, when the pressuredrops, in one of these separate pipes, below a value preset in terms ofthe minimum speed of the gases needed at the outlet of the injectors inorder to prevent the flash back, the intermediate valve mounted in thefuel gas supply pipe led off the separate pipe supplying the injector oflarger cross-section is caused to close, and subsequently also theintermediate valve, mounted in the separate pipe supplying the latter,the time lag occurring between these two closures being calculated insuch a manner that the composition of the fuel-comburent mixtureremaining in the portion of the separate injector of large cross-sectiondownstream of the mixing-measuring valve mounted in the latter, shall besufficiently depleted of fuel, prior to cutting the latter injector outof service, to prevent the burning of such mixture and so that, whensubsequentlY an increase of the flow is again required, the intermediatevalve mounted in the separate pipe supplying the large cross-sectioninjector shall be opened and that subsequently takes place the openingof the corresponding intermediate valve mounted in the fuel gas feedpipe at the time when the pressure ruling in such separate pipe shallexceed the said minimum value, and lastly, in such a manner that, whenthe large section injector has been cut out of service and the velocityof the gases at the outlet of the injector of smaller cross-sectionshall be less than the velocity required to prevent the flash back, theintermediate valve mounted in the fuel gas supply pipe led off thesupply pipe of the injector with the smaller cross-section shall befirst closed and subsequently the said main valve and/or the remainingintermediary valve shall also be closed, thus cutting the injector ofsmaller cross-section also out of service.
 11. A dilution burneraccording to claim 6, characterized in that the combustion chambercommunicates with the dilution chamber by means of a throttleterminating in a port having a diameter less than that of the saidoutlet opening provided in the terminal element and coaxial with suchopening, so that the unit comprising such throttle, such opening and thespace of the dilution chamber extending between the port and the openingprovides an injector permitting to establish in such space, thanks tothe considerable kinetic energy of the combustion gases escaping fromthe combustion chamber through such port, a depression sufficient toensure that the said dilution gases shall be drawn by the gases ofcombustion through the said outlet opening into the furnace whereon ismounted the burner.
 12. A dilution burner according to claim 11,characterized in that it comprises several dilution gas supply ductsterminating in the dilution chamber in locations uniformly distributedaround the said port through which the combustion gases are admitted insuch chamber.
 13. A dilution according to claim 11, characterized inthat the centres of the said port and opening are located in thelongitudinal axis of the burner.
 14. A dilution burner according toclaim 6, characterized in that the said terminal element comprises atleast one channel passing right through such element, permitting thefurnace to be heated to communicate with the dilution chamber, thusmaking it possible to draw through such channel gases from the furnaceinto the dilution chamber, such gases thus providing dilution gases forthe gases originating from the combustion chamber.
 15. A dilution burneraccording to claim 9, characterized in that the means provided to modifyin the portion of the feed pipe to the injector having the largestcross-section the composition of the fuel-comburent mixture, prior tocutting such injector out of service, comprise a pipe common for bothinjectors supplying the comburent gas for the latter and wherein ismounted a main valve, such common pipe being forked downstream of suchvalve in order to provide separate pipes for the two injectors, a pipefor the fuel gas supply being led off each of these separate pipes bymeans of a mixing-measuring valve of the comburent and fuel gases, whilean intermediate valve is provided for each of these separate pipes aswell as in each of the pipes led off the latter upstream of themixing-measuring valves, these separate pipes terminating each at one ofthe injectors, means for controlling said different valves withreference to the pressure ruling in the separate pipes downstream ofsuch mixing-measuring valves in such a manner that, when the pressuredrops, in one of these separate pipes, below a value preset in terms ofthe minimum speed of the gases needed at the outlet of the injectors inorder to prevent a flash back, the intermediate valve mounted in thefuel gas supply pipe led off the separate pipe supplying the injector oflarger cross-section is caused to close, and subsequently also theinterMediate valve, mounted in the separate pipe supplying the latter,the time lag occurring between these two closures being calculated insuch a manner that the composition of the fuel-comburent mixtureremaining in the portion of the separate injector of large cross-sectiondownstream of the mixing-measuring valve mounted in the latter, shall besufficiently depleted of fuel, prior to cutting the latter injector outof service, to prevent the burning of such mixture and so that, whensubsequently an increase of the flow is again required, the intermediatevalve mounted in the separate pipe supplying the large cross-sectioninjector shall be opened and that subsequently takes place the openingof the corresponding intermediate valve mounted in the fuel gas feedpipe at the time when the pressure ruling in such separate pipe shallexceed the said minimum value, and lastly, in such a manner that, whenthe large section injector has been cut out of service and the velocityof the gases at the outlet of the injector of smaller cross-sectionshall be less than the velocity required to prevent the flash back, theintermediate valve mounted in the fuel gas supply pipe led off thesupply pipe of the injector with the smaller cross-section shall befirst closed and subsequently the said main valve and/or the remainingintermediary valve shall also be closed, thus cutting the injector ofsmaller cross-section also out of service.